It is widely expected that organic EL panels, display panels using organic EL elements, will come into wide use in the next generation of flat panel displays. The many superior features of organic EL elements, such as self-emission, high contrast, wide viewing angle, and high-speed responsiveness, would make a display device employing an EL element an attractive, valuable, high-quality display device.
Such an organic EL element is generally manufactured by depositing an organic material on a first electrode formed on a glass substrate, and further forming a second electrode over the organic material. When such an organic EL element is used in a full-color display, display operation in full color is realized by a method such as selective color application, in which different organic materials are deposited for R, G, and B colors, or white light mode, in which white-light-emitting material is deposited and the display operation in full color is achieved using RGB color filters.
In the white light mode method, the same material can be deposited for all pixels irrespective of color, while in the selective color application method, different materials are sequentially deposited for R, G, and B pixels using masks for R, G, and B colors, respectively. To form a high-definition panel, deposition must be performed with a high degree of precision, such that a high precision (highly accurate) mask is necessary.
In a typical manufacturing process, when an organic material is deposited, a substrate subjected to deposition (a glass substrate on which a first electrode is formed) is typically placed on an upper position, and a mask is provided directly below the substrate in a closely contacting manner. Then, the organic material is vaporized from below so that the organic material will adhere to the first electrode exposed through the openings of the mask to thereby form an organic EL element.
In such a process, because a typical high-definition mask is very thin, with a thickness on the order of several tens of micrometers, the mask will deform under its own weight. Such deformation of the mask will weaken adherence of the mask to the substrate subjected to deposition, hampering the uniformity of deposition of the organic material over the surface of the substrate. As a general solution, a method is usually employed in which deposition is performed while exerting a tension on the mask along a given direction. Deposition of an organic EL element is described in, for example, Japanese Patent Laid-Open Publication No. 2002-231449.
The manufacturing of higher definition displays in which the pitch of pixels of a same color are reduced to approximately 100-150 μm, as have now become common, has created a need for masks 5 on which rectangular openings each having a width w and a height h of several tens of micrometers are arranged with a pitch px along x direction and a pitch py along y direction, as shown in FIG. 1.
In the mask 5, unopened regions of the mask, the material other than mask openings 3, become smaller as definition of the mask 5 becomes finer.
The relationship between the minimal distance between the mask openings 3 along the x direction, dx, and the minimal distance between the mask openings 3 along the y direction, dy, can be expressed as dx=px−w and dy=py−h, respectively. As is evident from the equations, the minimal distances dx and dy become smaller as the pitches px and py of openings are reduced, such that w and h dimensions of the openings increase.
Because an unopened region defined by a width of the minimal distance dx along the y direction sustains a tension in the y direction, and an unopened region defined by a width of the minimal distance dy along the x direction sustains a tension in the x direction, smaller minimal distances dx and dy lead to larger deformation of the openings when tension is exerted on the mask, which in turn hampers efforts to ensure the uniformity of deposition.
On the other hand, to secure minimal distances dx and dy of sufficient length, the dimensions w and h of the openings may be reduced with reference to the pitches px and py of the openings. This, however, leads to decrease in the area of light emission with the result that it becomes necessary to increase a density of current provided to the organic EL element in order to obtain a desired amount of light emission. The increased density of current brings about an increase in a current load of the organic EL element, which presents a problem that the service life of the element is shortened.